The invention relates generally to gas flow calibration systems and in particular to a gas flow calibration system which measures the rate of change of pressure in a gas flowing into a container maintained in a substantially isothermal or temperature invariant condition. The rate of change of the pressure under isothermal conditions is substantially indicative of the rate of mass flow causing the pressure change.
It is important in the manufacture of mass flow controllers and mass flow meters to calibrate them. A variety of calibration schemes are known, all of which suffer from one or more drawbacks. For instance, currently, bell provers are often connected upstream or downstream of mass flow controllers or mass flow meters to determine the actual rate of flow through the mass flow controller or mass flow meter being calibrated. The bell prover is a volumetric device that is similar to a graduated cylinder and has a movable piston positioned therein. A liquid seal is effected either by an oil film between the movable piston element and the walls of the cylinder or in some instances by mercury trapped between the piston and the walls of the cylinder. The oil has a tendency to vaporize or back stream causing possible contamination in the device under test. While the vapor pressure of mercury is typically lower than oil resulting in less mercury being lost during each measuring cycle, the mercury likewise can back stream into the device under test and contaminate it. Both the oil and mercury back streaming are significant and under today's standards of cleanliness for the manufacture of semiconductor devices, any measurable amounts of contamination are often too much to be tolerated. Bell provers all suffer from the drawback that the back pressure, against which the piston moves, may be significant in certain circumstances. For instance, if the pressure on the outside of the piston is ambient pressure or one atmosphere and if a mass flow controller or mass flow meter is to be calibrated for its flow characteristics for a low vapor pressure material in vapor state, the flow controller or flow meter cannot exhaust into a volumetric device such as a bell prover with one atmosphere back pressure because the vapor would be condensed in the lines or possibly even in the device under test, due to the high pressure. As a result, it would be necessary to pump the back pressure of the bell prover down and possibly to increase the overall temperature of the system to maintain the material in the vapor state. Thus, it is apparent that bell provers suffer from a number of drawbacks.
A secondary transfer standard in the form of a previously calibrated mass flow meter may also be coupled with the mass flow controller or mass flow meter under test. However, such a standard is a secondary and not a primary standard and would be less accurate than a primary standard.
It may also be possible to measure the rate of change of pressure of gas flowing into or out of a closed system which rate of change of pressure may be related in part to the mass flow rate of the gas. However, when gases are being compressed flowing into reservoir, the compression is normally an adiabatic compression wherein the internal energy of the gas increases in part due to an increase in temperature of the gas as it is being compressed. Thus, since part of the energy has gone into pressure volume work and another part of the energy into the increase in temperature, the change in pressure in and of itself is not indicative of the flow rate of the gas.
An additional problem associated with prior systems is that the particular rate of rise pressure measuring systems assume that the gas or vapor was a perfect, that is, that the gas or vapor obeyed the equation EQU PV=nRT
Assuming for purposes of argument here that n the number of mols is equal to 1, the equation would ideally reduce to EQU PV=RT
However, for real gases and vapors there is an implicit factor associated with the equation which may be written as EQU PV=ZRT
Z is the so-called compressibility factor of the gases, a dimensionless factor that varies with the gas species and with pressure and temperature. It is well known that with certain gases such as sulfur hexafluoride and in particular with certain low vapor pressure vapors such as tungsten hexafluoride, the compressibility factor departs significantly from unity. Thus, in a conventional rate of pressure rise flow measuring device with both temperature and pressure changing, Z varies in a complicated way on a real time basis. That variability of the compressibility factor will lead to further errors in the mass flow that is determined from the rate of pressure rise.
What is needed is a method and apparatus providing for a highly accurate primary standard which will not contaminate the device to be calibrated such as a secondary or transfer standard, by back streaming; which is able to calibrate mass flow controllers or mass flow meters carrying low vapor pressure vapors, is independent of the changes in ambient pressure experienced by the calibrator and is not perturbed by compressibility effects causing the gas or vapor to depart from perfect gas behavior.